More punch MH-31

For the QO-100 setup I made a headset adapter (link) based on the Yaesu MH-31 Electret Condenser Mic Modification (link) which is giving a nice punch in SSB with very low SSB Microphone Gain on my FT817ND and FT818.

Based on that experience and advise from Stephan DM5TU (link) to modify my MH-31 microphone with a electred condenser mic, I decided that I wanted to give it a try. But instead of modifying my original MH-31 I ordered 2 MH-31 clones from AliExpress (link) to modify. 




Schematic taken from Universal-Radio (link) so not my schematic.


  Step 1 : take it apart. 


Step 2 : de-solder the microphone element wires.


Step 3 : flip PCB 


Step 4 : remove wires from switch, capacitors and resistor. 


Step 5 : use de-soldering wick to clean contacts. 



Note : the PCB design from this clone isn't as my original Yaesu MH-31, 


TODO : add photo of original MH-31 PCB. 


but the good news is that for this PCB you don't need to cut PCB tracks and can start installing components right away.



Step 6 : install capacitors and wire

  • 1nF on location R1
  • 47nF on switch contact to corresponding PCB pad
  • 1uF on switch contact to corresponding PCB pad (negative lead to PCB pad)
  • one wire to center contact to corresponding PCB pad. 


Step 7 : install wire between outer switch pads on bottom of PCB and 8K2 resistor as shown below (between C pad connected to center of switch and pin 6 of RJ45 connector).

(check for shorts when done)

mh31elec-06 width=

Step 8 : to hold theElectret Condenser Mic I 3D designed a holder which has the same size as the original Dynamic microphone element.

I used part of a kitchen sponge, used scissors to it to shape, and use my soldering iron to burn a hole in it to hold the element. Soldered 2 wires to the element and pulled the wires through the sponge (please be sure to use the correct polarity) 


 Step 9 : place sponge with microphone element in 3D printed holder and glue the 3D printed holder to the PCB (blue = ground, yellow = mic signal). 


Ready for testing.


I setup the SSB-MIC-Gain on my FT818 to 30 and the rig is now giving a nice strong signal on the QO-100 without having to talk hard.